A power supply voltage which can be supplied to an LSI circuit (Large Scale Integrated circuit) is reduced because a recent process is miniaturized. A day on which only about 1 V can be supplied is near in future. Moreover, in recent years, a circuit has been requested which can operate even if the number of batteries is decreased because electronic equipment becomes portable.
Particularly, the above mentioned becomes a serious problem for an analog circuit for handling continuous signals. The representative of analog circuits is an operational amplifier. It is not too much to say that whether an operational amplifier can operate at a low voltage holds the key on advisability of change of an analog circuit to a low voltage.
FIG. 8 shows a conventional operational amplifier to be operated by a low-voltage power supply.
As shown in FIG. 8, this operational amplifier has a differential input portion 110 constituted of MOS transistors M101 to M103, a differential input portion 120 constituted of MOS transistors M104 to M106, a summing portion 130 constituted of MOS transistors M107 to M114, and an output portion 150 constituted of MOS transistors M115 and M116, a resistor R100, and a capacitor C100.
Moreover, the operational amplifier has a non-inversion input terminal 100, an inversion input terminal 101, bias terminals 102, 103, and 107 for supplying a bias voltage to gates of MOS transistors M103, M106, M113, M114, and M116 respectively operating as a current source, bias terminals 104 and 105 for supplying a bias voltage to gates of MOS transistors M109, M110, M111, and M112 respectively functioning as a cascode MOS transistor, and an output terminal 106.
In FIG. 8, a circuit constituted of the MOS transistors M104 to M116 omitting the MOS transistors M101 to M103 is a folded cascode operational amplifier using an N-type MOS transistor as an input transistor which has been well-known so far.
Moreover, in FIG. 8, a circuit constituted of the MOS transistors M101 to M103 and MOS transistors M107 to M116 omitting the MOS transistors M104 to M106 is a folded cascode operational amplifier using a P-type MOS transistor as an input transistor which has been well-known so far.
Therefore, the operational amplifier in FIG. 8 can be regarded as a circuit constituted by combining a folded cascode operational amplifier using a P-type MOS transistor as an input transistor and a folded cascode operational amplifier using an N-type MOS transistor as an input transistor.
In this case, when taking out the folded cascode operational amplifier using the P-type MOS transistor as an input transistor from the operational amplifier shown in FIG. 8, the circuit shown in FIG. 9 is obtained. In FIG. 9, 140A denotes a cascode power supply portion and 140B denotes a current mirror portion.
Then, the operation of the operational amplifier shown in FIG. 8 is described below.
In general, a relation between input voltage Vin of an N-type MOS transistor and current Ids flowing between a drain and a source can be shown by the following expression.Ids=(W/L)·μ·Cox(Vin−Vs1−Vthn)2  (1)
In this case, W denotes the channel width of a MOS transistor (MOSFET), L denotes the channel length of the transistor, μ denotes mobility, Cox denotes the capacity for unit area, Vs1 denotes the source voltage of the MOS transistor, Vthn denotes a threshold voltage.
According to the expression (1), no current flows unless the input voltage Vin has a value larger than Vs1+Vthn in the case of an N-type MOS transistor. That is, in this case, the N-type MOS transistor is turned off and normal operation cannot be performed.
Similarly, a relation between input voltage Vin of a P-type MOS transistor and current Ids flown through the P-type MOS transistor is shown by the following expression.Ids=(W/L)·μ·Cox(Vs2+Vthp−Vin)2  (2)
In this case, W denotes the channel width of a MOS transistor, L denotes the channel length of the transistor, μ denotes mobility, Cox denotes the electrostatic capacity for unit area, Vs2 denotes the source voltage of the MOS transistor, and Vthp denotes a threshold voltage.
According to the expression (2), no current flows unless the input voltage Vin has a value smaller than Vs2+Vthp in the case of a P-type MOS transistor. In the case of the enhancement type PMOS which is normally used, the symbol of Vth is negative.
Then, in the case of the operational amplifier shown in FIG. 8, an input signal voltage and an operable range are described below by referring to FIGS. 10A to 10C.
In FIGS. 10A to 10C, Vss denotes a lower (low-potential side) power supply voltage and Vdd denotes an upper (high-potential side) power supply voltage. The hatched portion in FIG. 10A shows that an N-type MOS transistor does not operate and the hatched portion of FIG. 10B shows that a P-type MOS transistor does not operate. Moreover, in the case of FIG. 10C, a hatched portion is formed in a range in which the N-type MOS transistor or P-type MOS transistor does not operate. However, because either of the N-type MOS transistor and P-type MOS transistor operates without fail, in accordance with FIGS. 10A and 10B, an input signal always operates from a positive power supply to a negative power supply.
Then, an operation range when lowering by a power supply voltage without changing the sum (Vth+Vs) of the threshold voltage Vth and source voltage Vs of a MOS transistor is described by referring to FIGS. 11A to 11C.
The hatched portion of FIG. 11A shows a portion where an N-type MOS transistor does not operate and the hatched range of FIG. 11B shows a portion where a P-type MOS transistor does not operate. Because both power supply voltages are lowered compared to the case of FIGS. 10A to 10C, not-hatched portions where a transistor can operate are decreased though the widths of hatched portions are the same. FIG. 11C shows the range in which the both do not operate by the hatched portion. This range in which the both do not operate is formed at a portion close to the center of the power supply voltage.
Therefore, when the sum of threshold values of the P-type and N-type MOS transistors exceeds the power supply voltage, it is impossible to operate an operational amplifier.
To solve the above problem, it is also considered to lower the threshold value, for example, make the threshold voltage Vthn negative and realize the so-called depletion type.
In this case, however, the Vs1 serving as a source voltage becomes higher than the input voltage Vin as understood from the expression (1). When the input voltage is equal to or close to the power supply voltage Vdd, no operation can be performed because the source voltage becomes higher than the power supply voltage Vdd. That is, in FIG. 10A, a hatched portion, that is, a region where no operation is performed is formed at the power supply voltage Vdd side, the operation region of operational amplifier is not improved even if changing the symbol of the threshold voltage Vthn.
Moreover, it is considered that operation is actually performed by setting the threshold voltage Vthn to a value close to zero. However, control of the absolute value of the threshold voltage Vthn is not easy and fluctuation of ±0.1 to 0.2 actually occurs. Furthermore, because the threshold voltage Vthn is fluctuated by temperature, a method of setting the threshold voltage Vthn to zero is not a realistic solution.
Therefore, to solve the above trouble, a method of securing an operation range by using a level shifter circuit is proposed in “J. Francisco Duque-Carrilo, L. Ausin Torelli, Jose M. Valverde, Miguel A. Dominguez, IEEE Journal of Solid-State Circuits, Vol. 35, No. 1, January 2000, p. 33”.
Moreover, a method of widening the dynamic range of an operational amplifier by controlling a well potential is disclosed in JP5-102756A.
However, the method in the above document has a problem that input current is not zero. When there is input current, it is impossible to apply the method to a switched capacitor circuit or it is necessary to add a resistance to an input terminal like the case of a bipolar circuit. When the resistance value is large, a problem occurs that offset becomes large. Moreover, when the resistance value is small, the resistance of the whole circuit is restricted to a small value and an operational amplifier to be used has a trouble that a capacity for driving the small resistance is applied.
However, the method according to the above patent gazette has a problem that the well potential can select only the binary value of either of source potential and power supply. Therefore, a threshold value to be realized through control is only a binary value and the versatility of the obtained threshold value is narrow. Moreover, a switching control signal for selecting is necessary, a trouble occurs that the application of operational amplifier is restricted.
Therefore, it is an object of the present invention to provide an operational amplifier which performs the rail-to-rail operation even for a low-voltage power supply and in which input current is zero.